1. Field of the Invention
The invention relates to a separator passage structure of a fuel cell. More particularly, the invention relates to a separator passage structure of a proton-exchange membrane fuel cell.
2. Description of the Related Art
A unit cell of a proton-exchange membrane fuel cell is constituted of a stack body formed by stacking a membrane-electrode assembly (i.e., MEA) and a separator. The MEA includes an electrolyte membrane formed of an ion-exchange membrane, an electrode (i.e., anode, fuel electrode) formed of a catalyst layer which is provided on one surface of the electrolyte membrane, and another electrode (i.e., cathode, air electrode) formed of another catalyst layer which is provided on the other surface of the electrolyte membrane. A diffusion layer is provided between the MEA and the separator. On the separator, a passage for supplying fuel gas (i.e., hydrogen) to the anode, a passage for supplying oxidizing gas (i.e., oxygen, generally, air) to the cathode, and a passage through which a refrigerant (generally, cooling water) passes are formed. A module includes at least one unit cell. A cell stack body is formed by stacking the modules. A terminal, an insulator, and an end plate are provided at each of both ends of the cell stack body in a direction in which cells are stacked (hereinafter, referred to as a “cell stacked direction”). The cell stack body is fastened in the cell stacked direction by using a fastening member (e.g., a tension plate), which is provided outside the cell stack body and which extends in the cell stacked direction, whereby a fuel cell stack is formed. In the proton-exchange membrane fuel cell, the reaction which changes hydrogen to a hydrogen ion and an electron occurs on the anode side, and the hydrogen moves to the cathode side through the electrolyte membrane. The reaction which generates water from oxygen, the hydrogen ion, and the electron (the electron generated on the anode side of the adjacent MEA moves to the cathode side through the separator) occurs on the cathode side.
Anode side: H2→2H++2e−
Cathode side: 2H++2e−+(1/2)O2→H2O.
A concave groove and a convex rib are formed on the separator. The concave groove on a surface of the separator which faces the MEA constitutes a gas passage through which reaction gas of the fuel gas or the oxidizing gas passes. The convex rib contacts the diffusion layer, and constitutes a conductive passage. Since the reaction gas is consumed by the power generating reaction, the concentration and the partial pressure of the reaction gas decrease toward the downstream side, and the gas flow speed is then reduced. Also, due to the water generated by the power generating reaction, the possibility that clogging occurs due to moisture in the diffusion layer and the gas passage increases toward the downstream side. Accordingly, it is necessary to prevent the gas flow speed on the downstream side from being reduced. In order to satisfy this condition, Japanese Patent Laid-Open Publication No. 11-16590 discloses a separator passage structure for maintaining the flow speed of the reaction gas by reducing the groove width of the gas passage or by reducing the groove depth.
However, the conventional separator passage structure of the fuel cell has the following problems.
1) When the cross sectional area of the gas passage is changed by changing the gas passage width, the width of the contact area of the electrode with the separator convex rib changes. Therefore, it becomes impossible to maintain the homogeneity of the reaction in the entire cell.
2) When the cross sectional area of the gas passage is changed by changing the depth of the gas passage groove, it is necessary to uniform the thickness of the separator to the thickness of the portion where the gas passage is the deepest in the entire cell surface. (This is because, if the thickness of the separator is changed in the direction perpendicular to the cell stacked direction, the stack is bent when the cells are stacked. Accordingly, the thickness of the separator needs to be constant.) Therefore, the thickness of the separator itself increases, and the entire length of the stack therefore increases. Particularly, in the case of a metal separator, since groove depth is limited due to the limitation on press. Accordingly, when the cross sectional area of the gas passage is changed by changing the depth of the gas passage groove, the amount of change is limited.